Except that's not how it always works: About half of all unintended pregnancies are the result of contraceptive failure, where a condom breaks or birth control pills aren't taken at the right time. The least expensive methods of contraceptive tend to be the least effective.

That got a team of researchers at Washington University wondering what would happen if women had access to all contraceptives at no cost. IUDs, for example, are about 20 times more effective than birth control pills - but also tend to be significantly more expensive.

Over the course of three years, they gave over 9,000 women in the St. Louis area access to free contraceptives. Study participants could choose from birth control pills or more long-acting contraceptives, like the implantable IUD. Three in four women chose the latter.

The researchers published their results Thursday and saw some dramatic differences between those in the study, and those outside of it.

Teen pregnancies - 80 percent of which are unintended - plummetted. They stood at 6.3 per 1,000 teens in the study group, compared to 34 per 1,000 teens nationally.

Abortion rates were significantly lower, too. In the St. Louis area, 13.4 per 1,000 women had an abortion in 2010. Among the women involved in this study, the rate stood at 5.9 per 1,000 women.

The study authors attribute a lot of those differences to the widespread use of long-acting contraceptives. Such birth control - used by 75 percent of the women in this study - is only used by 8.5 percent of women nationally.

The Affordable Care Act will expand this experiment nationally by making contraceptives no-cost for all insurance subscribers. The study authors estimate that could have a marked effect on abortion rates, "preventing as many as 41–71% of abortions performed annually in the United States."